Technical Field
The invention relates to printing. More particularly, the invention relates to the measuring of qualities of a printed sheet, for example, reflectance excluding specular reflectance, reflectance including specular reflectance, e.g. gloss, transmittance, and half-tone coverage.
Description of the Background Art
Many factors affect the qualities of an image that is printed on a sheet. Such phenomena as reflection and transmittance of light occur because the frequencies of the light waves do not match the natural frequencies of vibration of the objects. When light waves of these frequencies strike an object, such as a sheet of paper, the electrons in the atoms of the object begin vibrating. But instead of vibrating in resonance at a large amplitude, the electrons vibrate for brief periods of time with small amplitudes of vibration; then the energy is reemitted as a light wave. If the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted. If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material. Rather the electrons of atoms on the material's surface vibrate for short periods of time and then reemit the energy as a reflected light wave. Such frequencies of light are said to be reflected.
The color of the objects that we see is largely due to the way those objects interact with light and ultimately reflect or transmit it to our eyes. The color of an object is not actually within the object itself. Rather, the color is in the light that shines upon it and is ultimately reflected or transmitted to our eyes. The visible light spectrum consists of a range of frequencies, each of which corresponds to a specific color. When visible light strikes an object and a specific frequency becomes absorbed, that frequency of light never makes it to our eyes. Any visible light that strikes the object and becomes reflected or transmitted to our eyes contributes to the color appearance of that object. Thus, the color is not in the object itself, but in the light that strikes the object and ultimately reaches our eye. The only role that the object plays is that it might contain atoms capable of selectively absorbing one or more frequencies of the visible light that shine upon it. If an object absorbs all of the frequencies of visible light except for the frequency associated with green light, then the object appears green in the presence of visible light. If an object absorbs all of the frequencies of visible light except for the frequency associated with blue light, then the object appear blues in the presence of visible light.
Reflectivity is a directional property. Most surfaces can be divided into those that give specular reflection and those that give diffuse reflection. For specular surfaces, such as glass or polished metal, reflectivity is nearly zero at all angles except at the appropriate reflected angle. That is, reflected radiation follows a different path from incident radiation for all cases other than radiation normal to the surface. For diffuse surfaces, such as matte white paint, reflectivity is uniform; radiation is reflected in all angles equally or near-equally. Such surfaces are said to be Lambertian. Most real objects have some mixture of diffuse and specular reflective properties.
Gloss is an optical property describing the ability of a surface to reflect light into the specular direction. The factors that affect gloss are the refractive index of the material, the angle of incident light and the surface topography. Gloss is one of the factors that describe the visual appearance of an object. Factors that affect gloss include, for example, the refractive index of the material, the angle of incident light relative to the surface of the material, and the material's surface topography. Very rough surfaces, such as chalk reflect no specular light and appear dull. Gloss is also expressed as luster in mineralogy, or sheen in certain fields of application.
The appearance of gloss depends on a number of parameters which include the illumination angle, refractive index, surface condition, and observer characteristics. Primarily light is reflected from a surface in one of two ways. In specular reflection, the angle of the light reflected from the surface is equal and opposite to the angle of the incident light. A diffuse reflection scatters the incident light over a range of directions. Variations in surface texture influence specular reflectance levels. Objects with a fine surface texture, i.e. highly polished and smooth, allow a high percentage of light to be reflected from their surfaces making them appear shiny to the eye. This is due to a greater amount of incident light striking the surface being reflected directly back to the observer; the majority of which being reflected in the specular direction.
Conversely, objects with rough surfaces cause the light to be deflected at different angles according to the surface profile resulting in a scattering of light away from the angle of reflection. This causes the object to appear dull or matte. The image forming qualities are much lower making any reflection appear blurred. The higher the degree of surface roughness, the greater the scattering of light resulting in a lower gloss level.
Due to the refractive index, the type of substrate material also has an important effect on the amount of specular reflection from its surface. Nonmetallic materials such as dielectrics and insulators, i.e. plastics and coatings, produce a higher level of reflected light when illuminated at a greater illumination angle due to light being absorbed into the material or being diffusely scattered depending on the color of the material. Metals, e.g. conductors, do not suffer from this effect producing higher amounts of reflection at any angle than nonmetals
Further, many different phenomena influence the reflection spectrum of, for example, a color halftone patch printed on a diffusely reflecting substrate, e.g. paper. These phenomena comprise the surface reflection at the interface between the air and the paper, light scattering and reflection within the substrate, i.e. paper bulk, and the internal reflections at the interface between the paper and the air. The lateral scattering of light within the paper substrate and the internal reflections at the interface between the paper and the air are responsible for what is generally called the optical dot gain. In addition, due to the printing process, the deposited ink surface coverage is generally larger than the nominal coverage, yielding a physical or mechanical dot gain. Effective ink surface coverage depend on the inks, on the paper, and also on the specific superposition of the different inks.
As can be seen from the foregoing, the physics of light in connection with the printing of an image on a sheet is complex. It would be advantageous to be able to measure any one or more of the foregoing qualities quickly and accurately in situ and use the results of such measurements to enhance the process of printing in both real time and to parameterize the printing process.